1. Field of the Invention
This invention relates to a signal analyzer apparatus with an automatic frequency measuring function and, more particularly, to a signal analyzer apparatus for analyzing signal characteristics in a frequency region, upon sweeping of the signal in a wide frequency band, so that a frequency at a desired point of the characteristics can be measured with high precision.
In addition, this invention relates to a signal analyzer apparatus for facilitating frequency measurement of a signal by utilizing the technique disclosed in U.S. Pat. No. 4,611,164, entitled "Spectrum Analyzer With Automatic Peak Frequency Tuning Function", which is assigned to the present applicant.
2. Description of the Related Art
FIG. 1 illustrates the arrangement of a conventional spectrum analyzer (composed of those elements surrounded by solid lines) used for frequency measurement. FIGS. 2A to 2F show display examples of obtained by a conventional technique. The operation of this frequency measurement will now be described below.
Referring to FIG. 1, frequency controller 22 receives desired measuring frequency information (e.g., 0 to 100 MHz) and then commands sweep signal generator 20 to output to local oscillator 2 a sweep voltage corresponding to the desired measuring frequency information. Local oscillator 2 then outputs a signal, having a frequency proportional to the sweep voltage, to mixer 1. Mixer 1 mixes the output signal from local oscillator 2 with an input signal (0 to 100 MHz) to be measured, and converts the resultant signal into an intermediate-frequency (IF) signal. The IF signal is then selected by a predetermined resolution bandwidth in IF circuit 4, and is detected by detector 5. The detected signal is converted into a digital signal by A/D converter 6, after which the digital signal is stored in memory 7, in accordance with the frequency of the input signal to be measured, and is displayed by display unit 8. FIG. 2A shows such a display example. A marker (a black dot on the display) is set by marker setting section 24 at a desired spectrum point of the data displayed in this manner. Frequency information fm (70 MHz in FIG. 2A) at this time is supplied to frequency controller 22. Frequency controller 22 causes sweep signal generator 20 to perform sweep measurement by setting the center frequency and the frequency measuring width (f2-f1) in the example shown in FIG. 2A to be frequency fm and zero, respectively, in the example in FIG. 2F. In this state, frequency measuring section 21 additively measures output frequencies A and B from local oscillator 2 and IF circuit 4, and supplies its output signal to display unit 8, to numerically display the output signal as frequency information (A+B) of the input signal to be measured. Note that this value, i.e., (A+B) is obtained when an IF signal is obtained by a beat-down operation of mixer 1. When on the other hand, mixer 1 performs a beat-up operation, this value becomes (A-B).
In such frequency measurement, the relationship between the frequency resolution of a spectrum (to be determined by the band of IF circuit 4 and referred to as an RBW hereinafter) and the resolution of data processing must be considered. For example, in the example shown in FIG. 2A, the frequency measuring width (to be referred to as a span hereinafter) is 100 MHz (=f2-f1). Assuming that data is processed and displayed by dividing this span into 500 points on the axis of abscissa (frequency axis), then, the resolution of data processing is (2 MHz/1 point). In this case, if RBW=50 kHz, signals plotted between measuring points cannot be measured. In order to prevent this, local oscillator 2 is swept in an analog manner, so that the peak value of a signal between points is obtained upon detection, and is displayed at a predetermined point. Therefore, in the case of FIG. 2A, all input signals within the range of 0 to 100 MHz can be measured. However, a maximum frequency error becomes 2 MHz.
For this reason, in order to accurately measure a frequency, frequency tuning must be performed. For this purpose, the following condition must be satisfied: EQU RBW.gtoreq.span.times..alpha.
for .alpha.=0.02 to 0.05. FIGS. 2A to 2F show the display examples when frequency tuning is performed. As shown in FIG. 2B, the signal to be measured is magnified and measured by setting frequency fm (70 MHz), which is determined when the marker point is set at the desired spectrum in FIG. 2A, as center frequency fc, and reducing the span in FIG. 2A to 10 MHz (f2-f1). At this time, the marker is again set at the peak of the spectrum and frequency fp (=fm=70.85 MHz) corresponding to the peak is measured. Then, as shown in FIG. 2C, the spectrum is displayed by setting frequency fp as the center frequency. In addition, the span is reduced to 1 MHz, as shown in FIG. 2D, and frequency fp (70.75 MHz) at the peak point of the spectrum is measured. Then, as shown in FIG. 2E, the spectrum is displayed by setting frequency fp as the center frequency. In this case, in order to satisfy the above-described measuring condition, i.e., RBW=50 kHz.gtoreq.span.times..alpha.=1 MHz.times.0.05, the span is set to be zero, as shown in FIG. 2F. In this state, the frequency is measured by frequency measuring section 21. The span is gradually reduced in this manner, since a target spectrum may fall outside a display screen if the span is reduced at once.
The above-described operation has been performed manually. In the operation, a technique of searching for the peak value of the spectrum, sequentially reducing the span, and performing measurement in a desired span is disclosed in the above-described U.S. Pat. No. 4,611,164. This technique is realized by adding peak search section 23 encircled by the dotted line in FIG. 1.
In such a conventional technique, the following problems arise:
(i) The above-described manual operation is very cumbersome, and a considerably long period of time is required for frequency measurement. Since an operation is performed while measuring conditions such as a span and RBW are considered and a screen is monitored, an operation error tends to be caused.
Although the technique described in U.S. Pat. No. 4,611,164 can partially assist the manual operation above described, it has nothing to do with frequency measurement with an RBW considered.
(ii) According to any conventional technique, a display screen must be changed for a simple operation of accurately measuring a frequency. Therefore, when measurement of the next cycle is to be performed with an initial screen state again, the initial screen must be recorded and initial values must be set again manually.